This invention relates to the field of switches, and more particularly to the field of floatable switches designed to sense a liquid level and operate a switch in response to the same.
The problem of determining a liquid level and taking action based on that data can be seen in a number of settings. The familiar toilet tank, for example, includes a float switch of the pivoting lever type, in which a buoyant object is fixed to one end of a lever arm. The other end of the lever is pivoted about a fixed point, and action can be taken by the lever itself (as seen in the toilet example, where the lever operates the water cutoff switch) or by an orientation-sensitive device within the float, as shown in U.S. Pat. No. 3,090,849 Coulin, May 21, 1963.
A primary disadvantage of the lever arm approach is the space required to accommodate the lever. The art has thus introduced a free-floating switch, retained in place by the tethering action of a flexible device such as a power cord.
Regarding the switch itself, the classic solution to the orientation-sensitive switch is the mercury switch, in which a quantity of mercury is contained within a vessel into which electrical conductors are inserted. The mercury moves within the vessel, and at certain vessel orientations the mercury overlies both conductors, completing the electrical circuit. The hazards of working with mercury have reduced the popularity of this type of switch.
Design objectives for an electro-mechanical switch include reliability and simplicity. An important factor in gaining reliability is freedom from contact arcing, which occurs when the switch contacts are in close proximity. The difficulty here is that the changes in liquid level sensed by a float switch are relatively slow. If a switch's contact spacing were directly proportional to the liquid level, arcing would occur for an unacceptably large portion the operating range. It is desirable, therefore, to achieve a "snap" action, in which the switch contacts are rapidly moved from full mutual contact to a position of wide separation. A typical embodiment of this concept is seen in U.S. Pat. No. 4,692,576 Frede, Sep. 8, 1987, in which a conventional switch, operated by a lever arm, is encapsulated within a float housing. A ball is provided within the housing to operate the switch lever by moving with a chamber. The chamber has two portions, divided by a ledge, which prevents the ball from moving until the float reaches a predetermined angle. Another ball operated device is shown in U.S. Pat. No. 4,644,117 Grimes, Feb. 17, 1987, in which the ball's movement is restrained by magnets rather than by a ledge.
Similarly, U.S. Pat. No. 4,755,640 Cooley, Jul. 5, 1988 utilizes a weight travelling back and forth on a post. Here the lever arm includes a cam follower portion that rides on the surface of the weight to operate the switch. According to the disclosure, snap action is achieved by the static friction between the weight and the post.
All of these device possess significant drawbacks. First, they all use a number of preassembled commercial components. All use at least a commercial switch mechanism, and the '117 disclosure includes a complex assemblage of parts. Such mechanisms introduce higher cost and greater complexity into the design. Further, the reliability of these designs seems to flow directly from their complexity. The sliding weight of the '640 patent offers only limited precision and repeatability, as it depends on the static friction between components, which can vary greatly. On the other hand, the reliability demonstrated by the other devices is only achieved at a price of high cost and complexity.
The art has yet to produce a float switch that offers a combination of simplicity and reliability. Those objectives are realized in the present invention.